Chapter 20
Inheritance, Genetics, and Molecular Biology
So how does inheritance work?

Inheritance is governed by genes

Genes are present on chromosomes

Chromosomes are made of DNA

DNA is our genetic material
Genotype
 Genotype – particular combination of genes in a individual
 Alleles - alternate forms of a specific gene (e.g. allele for unattached earlobes
and attached earlobes)
o Dominant alleles will be expressed and will mask a recessive gene (Tt
or TT)
o Recessive alleles will only be expressed when both copies of the gene
are the recessive allele (tt)
 Homozygous dominant genotype - 2 dominant alleles (TT or AA)
 Homozygous recessive genotype - 2 recessive alleles (tt or aa)
 Heterozygous genotype - two different alleles (Tt or Aa)
Phenotype
Phenotype – the physical or outward expression of the genotype
Genotype
Phenotype
EE
unattached earlobes
Ee
unattached earlobes
ee
attached earlobes
Gregor Mendel
 Much of our understanding of basic genetics stems from the work of Gregor
Mendel, an Augustinian monk who worked with the common garden pea
 Mendel also tested the effects of tracking two different traits simultaneously
o Would the two traits segregate together or would they segregate
independently?
So how does inheritance work?
 Crossing-over makes new chromosomes with new combinations of alleles
Pedigrees are like genetic family trees
 Humans are unique genetic test subjects

Achondroplasia
o Affects about 1 in 25,000 people
o Head and torso develop normally but arms and legs are short

We often refer to these simple genetic (dominance vs. recessiveness) traits as
Mendelian traits
Other traits show non-Mendelian patterns of inheritance

Polygenic inheritance
 Polygenic traits - two or more genes govern one trait
 Each dominant allele codes for a product so these effects are additive
 Results in a continuous variation of phenotypes
 Environment can also affect the phenotype
o e.g. skin color ranges from very dark to very light
o e.g. height varies from very short to very tall
 Multifactorial trait – a polygenic trait that is particularly influenced by the
environment
o e.g. skin color is influenced by sun exposure
o e.g. height can be affected by nutrition
Codominance
o Occurs when two different alleles are equally expressed in a heterozygote
Multiple alleles
o The gene exists in several allelic forms
o A person only has 2 of the possible alleles
o A good example is the ABO blood system
o IA and IB are codominant alleles
o The i allele is recessive to both IA and IB; therefore to have type O
blood you must have 2 recessive alleles (ii)
Sex-linked inheritance
o Traits are controlled by genes on the sex chromosomes
o X-linked inheritance - the allele is carried on the X chromosome
o Y-linked inheritance - the allele is carried on the Y chromosome
o Most sex-linked traits are X-linked
X-linked disorders
o Much more common in males than females because recessive alleles are
always expressed in males
o Most X-linked disorders are recessive
o Color blindness - most common is red-green color blindness
o Muscular dystrophy - characterized by wasting of muscles and death
by age 20
o Hemophilia - characterized by the absence of particular clotting
factors that causes blood to clot very slowly or not at all
Incomplete dominance
o Occurs when the phenotype of the heterozygote is intermediate between the
two homozygotes
Environmental influences on phenotype
The function of DNA
o It must store an amazing amount of genetic information
o It must be replicated in order to be passed on to the next generation
o It must replicate faithfully, but not too faithfully
o It must give rise to mutations to provide genetic diversity
RNA structure and function
o Single-stranded
o Composed of covalently-attached nucleotides
o Sugar-phosphate backbone
o The bases are A, C, G, and uracil (U)
o Three types of RNA
o Ribosomal (rRNA) joins with proteins to form ribosomes
o Messenger (mRNA) carries the genetic information from DNA to the
ribosomes
o Transfer (tRNA) transfers the appropriate amino acids to a ribosome
where they are added to a growing polypeptide
Proteins
o Composed of subunits of amino acids
o The sequence of amino acids determines the shape of the protein
o Synthesized on the ribosomes
o Important for diverse functions in the body including hormones, enzymes,
and transport
2 steps of gene expression
o Transcription – DNA is transcribed to make a mRNA in the nucleus of our
cells
o Translation – mRNA is translated to make a protein in the cytoplasm
1. Transcription
o mRNA is made from a DNA template
o mRNA is processed before leaving the nucleus
o mRNA moves to the ribosomes to be read
2. Translation
o Initiation - mRNA binds to the small ribosomal subunit and the two
ribosomal units come together
o Elongation - the polypeptide lengthens
o Each tRNA picks up a particular amino acid
o Each tRNA has an anticodon that is complementary to the codon on the
mRNA
o The tRNA anticodon binds to the codon and drops off an amino acid to the
growing polypeptide
o Termination - a stop codon on the mRNA causes the ribosome to fall off the
mRNA
DNA technology terms
o Genetic engineering – altering DNA in bacteria, viruses, plants, and/or animal
cells through recombinant DNA technology
o Recombinant DNA – contains DNA from 2 or more different sources
o Transgenic organisms – organisms that have a foreign gene inserted into
them
o Biotechnology – using natural biological systems to create a product or to
achieve an end desired by humans
DNA technology
o Gene cloning through recombinant DNA
o Polymerase chain reaction (PCR) to amplify target DNA
o DNA fingerprinting for forensic identification
o Biotechnology products from bacteria, plants, and animals
o Geneology
o Genetic analysis for personalized medicine
PCR (Polymerase chain reaction)
o Used to clone small pieces of DNA
o Important for amplifying DNA for analysis such as in DNA fingerprinting
Gene cloning
o Recombinant DNA – contains DNA from 2 or more different sources that
allows genes to be copies
o An example using bacteria to clone the human insulin gene
o Restriction enzyme – used to cut the vector (plasmid) and the human
DNA with the insulin gene
o DNA ligase seals together the insulin gene and the plasmid creating
recombinant DNA
o Bacterial cells take up the plasmid and the gene is copied and product
can be made
DNA fingerprinting
o Fragments are separated by their charge/size ratios
o Results in a distinctive pattern for each individual
o Often used for paternity or to identify an individual at a crime scene or
unknown body remains